Vertical migration and transformation of metal(loid)s in surface layers of tailings ponds driven by induced biocrusts: Emphasizing arsenic and zinc dynamics

Abstract

The formation of biological soil crusts (biocrusts) constitutes a pivotal initial phase in the natural rehabilitation of heavy metal(loid)-contaminated tailings ponds. Through integrating field studies and culture experiments, we investigated metal(loid) transformation and vertical redistribution dynamics during biocrust succession. Artificially induced biocrusts exhibit distinct developmental timelines: algal crusts established within 20 days, whereas moss crusts exhibited rapid expansion after 40 days. Biocrust development significantly enhanced upward metal(loid) migration, leading to differential enrichment patterns — arsenic (62 %) accumulation predominated in algal crusts, while zinc (25 %) peaked in moss crusts. Metal(loid)s were chelated in tightly bound extracellular polymeric substances (EPS), and the formation of biocrusts increased the residual fraction while reducing bioavailability. With development of algal crusts, As(V) and As(III) were reduced to elemental arsenic. Microbial community analyses revealed compositional shifts, with EPS-producing taxa and Fe/Mn oxide-precipitating microorganisms becoming increasingly abundant. Concurrent molecular analyses identified upregulation of key functional gene involved in arsenic redox transformation and zinc uptake, alongside variations in photosynthetic systems. These findings have provided novel insights into biocrust-mediated biogeochemical cycling in polluted environments.